This invention relates to improvements in torque sensors, particularly for use in electric power assisted steering systems. It also relates to an electrically assisted power steering system that incorporates a torque sensor.
It is known to provide an electrically assisted power steering system in which an electric motor applies a torque to the steering that assists the driver in turning the wheel. In a typical column drive system this comprises a steering column shaft that is supported within a shroud. The shaft is connected to the steering wheel at one end and engages a gearbox at the other. The gearbox connects the shaft to an electric motor and also to the steered wheels of the vehicle. The electric motor is driven by a suitable drive circuit to apply the torque, and the drive circuit responds to a torque demand signal produced by a controller. A torque sensor is provided which measures the torque applied to the column shaft by the driver, or measures the torque at some other point in the system depending on the type of system produced. This torque measurement is fed into the controller and used as the basis for producing the torque demand signal. Generally speaking the higher the measured torque, the larger the value of the assistance torque.
The assistance torque applied by the motor can also vary as a function of vehicle speed or other vehicle operating parameters. Indeed the motor can also be used to apply other torques that may be helpful in controlling the vehicle.
A number of different torque sensors are available but commonly a torque sensor is used that works by measuring the differential angular displacement between the two ends of a torsion bar that is connected in line between the column shaft and the gearbox. The angular displacement can be measured using a range of different sensors, with magnetic or inductive type sensors being commonplace due to their relatively low production cost. As the sensors are well known they will not be described here in detail.
The torque sensor is generally housed within the gearbox. By securing the motor to the gearbox housing all of the electric assistance parts of the steering system can be manufactured as a single compact unit.
The torsion bar provides a mechanical link between the steering wheel and road wheels. To prevent damage to the torsion bar in the event that excessive torsional loads are applied (perhaps when the vehicle road wheels hit an obstacle at speed) it is known to provide drive dogs that engage to limit the amount of twist that the torsion bar will experience. This also provides a continued mechanical connection in the very unlikely event that the torsion bar fails, albeit that there will be some free play present.
In such an event, there is a risk that the steering column shaft could move axially and separate the two ends of the torque sensor so that the dogs will not engage when the steering wheel is rotated. In the applicants earlier systems this is prevented by providing a shoulder on the steering column shaft where it enters the gearbox housing that will engage with a corresponding shoulder on the gearbox or within the shroud that supports the steering shaft. This works well but does have assembly disadvantages in that it adds additional constraints to the design of the gearbox or shroud.